1. Field of the Invention
The present invention relates to the sulfuric acid electrolysis process which directly electrolyzes concentrated sulfuric acid by using the conductive diamond anode to form oxidizing agent stably.
2. Description of the Related Art
In the so-called wet washing technology, where silicon wafer works are objects of cleaning as seen in the semiconductor device manufacturing, persulfuric acid or persulfate is used as removing agent for used photoresist, metals and organic pollutants. These persulfuric acid or persulfate are known to form through the electrolysis of sulfuric acid, and already manufactured electrolytically on an industrial scale. (Patent Document 1)
Patent Document 1 discloses the method to produce ammonium persulfate through electrolyzing the electrolyte comprising aqueous ammonium sulfate solution. This method applies relatively low concentration of aqueous Sulfate solution at 30-44% by mass. However, electrolysis of the aqueous Sulfate solution at such relatively low concentration as shown in Patent Document 1 reveals a problem that the wash stripping efficiency of photoresist, etc. is low.
In order to solve this problem, the inventors of the present invention have invented, and filed for patent, the sulfuric acid electrolysis process to manufacture persulfuric acid by electrolyzing concentrated sulfuric acid using a conductive diamond electrode, as a technology to supply persulfuric acid with a high cleaning effect, continuously and quantitatively at a high efficiency, and a cleaning process for silicon wafer works applying persulfuric acid manufactured by said process. (Patent Document 2) Compared with platinum electrodes widely used so far as electrodes to form persulfate, this conductive diamond electrode, giving a larger oxygen generation overpotential, shows a higher efficiency in electrolytic oxidation of sulfuric acid into persulfuric acid, is superior in chemical stability and has a longer electrode life.
The process described in Patent Document 2 electrolyzes concentrated sulfuric acid at a concentration over 90% by mass, and the oxidizing agent formed from the electrolysis reaction of concentrated Sulfuric acid, such as peroxomonosulfuric acid, contains less moisture and therefore, is not decomposed through reaction with moisture, capable of stably forming such oxidizing agent as peroxomonosulfuric acid, achieving a high wash stripping efficiency for photoresist, etc.
However, concentrated sulfuric acid has such features derived from its high viscosity with less fluidity, compared with water or relatively thin aqueous solution, that when it is used as an electrolyte for electrolysis, the generated gas from the electrolysis is hard to be liberated from the electrode surface, and also bubbles formed by liberated gas in the electrolyte take time to diffuse and therefore, are difficult to be discharged outside the electrolytic cell. Accordingly, if such gas covers the electrode surface or is contained in the electrolyte plentifully, the resistance between the anode and the cathode increases, raising the cell voltage, which may eventually lead to a phenomenon that electrolytic current will not be supplied in excess of the maximums supply output of the power source, which interferes with the production process of persulfuric acid. Also, other substances than gas formed by electrolysis are easy to precipitate due to its small solubility in the concentrated sulfuric acid, especially at a low temperature. When precipitate, they will also become a factor to interfere with electrolytic current flow as with the case of gas.
In Patent Document 3, the sulfuric acid electrolysis process is disclosed, as a part of the sulfuric acid recycle type cleaning system, which produces persulfuric acid through electrolysis of concentrated sulfuric acid by using the conductive diamond anode. Patent Document 3 also discloses that the formation efficiency of persulfuric acid is raised by controlling the temperature of the solution to be subjected to electrolytic reaction in the range of 10-90 degree Celsius and the rate of dissolution of persulfuric acid solution of the photoresist is increased by controlling the concentration of sulfuric acid to 8M or above, but there is no disclosure about the relationship between the flow rate of the electrolyte and the electrolysis temperature, and neither disclosure nor suggestion are given about the means to perform the sulfuric acid electrolysis stably.
Meanwhile, such troubles have often happened that when in the sulfuric acid electrolysis process to manufacture persulfuric acid using the conductive diamond anode as described in Patent Document 2 and Patent Document 3, electrolytic current value is raised to operate the electrolysis cell, the cell voltage sharply rise beyond the limit of the connected rectifier within a short period of time and the set-up current value sharply descends, causing failure of electrolysis operation. In particular, such trouble of failure in electrolysis was significant when the concentration of concentrated sulfuric acid in said electrolysis was 70% by mass or more and the current density was 20 A/dm2 or more in said electrolysis.
Concentrated Sulfuric acid has a characteristic that its coagulation point varies with concentration; for instance, at 85.66% by mass the point is 7.1 degree Celsius, but at 94% by mass, −33.3 degree Celsius, at 100% by mass, 10.9 degree Celsius, and at 74.36% by mass, −33.6 degree Celsius. It is presumed that to a small variation of concentration, the property changes significantly, and that near the coagulation point, viscosity varies considerably and said troubles tend to easily occur. (Non-Patent Document 1, P. 5-7)
Also, according to Non-Patent Document 1, Pages 5-7, the viscosity of concentrated sulfuric acid is, for instance, 0.99 cP, at 10% by mass of concentration at 30 degree Celsius, being equal to water, but for a high concentration, the value is large, for instance, 7.9 cP at 70% by mass of concentration, 15.2 cP at 80% by mass of concentration, and 15.6 cP at 90% by mass of concentration. Also, the viscosity largely depends on temperature. The lower the temperature, the larger it tends to be. For instance, for 90% by mass of concentration, 31.7 cP at 15 degree Celsius, 23.1 cP at 20 degree Celsius, 15.6 cP at 30 degree Celsius, 11.8 cP at 40 degree Celsius, and 8.5 cP at 50 degree Celsius. In order to promote gas elimination in the region of a high sulfuric acid concentration, applied temperature must be raised, which, however, is known undesirable due to increased decomposition of persulfuric acid.    [Patent Document 1] Tokkaihei 11-293484 Patent Gazette    [Patent Document 2] Tokkai 2008-19507 Patent Gazette    [Patent Document 3] Tokkai 2006-278838 Patent Gazette    [Non-Patent Document 1] Handbook of Sulfuric Acid (published by Japan Sulfuric Acid Association-1968)